Size of a reflector antenna for a satellite or an array antenna having a plurality of unit antennas is more than ten times that of the unit antenna.
Therefore, a minimum distance Rfar between a source antenna and a measurement antenna must satisfy Rfar=2×L2/λ. Herein, L is a length of aperture of the measurement antenna, and λ is a wavelength of operation frequency.
For example, a minimum distance for measuring a far-distance radiation pattern of 20λ antenna, i.e., L=20λ, operating in frequency 10 GHz, is 24 m. Thus, the far-distance radiation pattern cannot be measured in an anechoic chamber having a smaller distance than 24 m.
For solving the above problem, there is an open-air measurement method. However, the open-air measurement method is sensitive to the weather, and occurs interferences and errors from conventional communication services. In addition, a military antenna and a security antenna can be exposed to a spy satellite, so that secrets cannot be secured.
In another alternative, there is a compact range method which generates a plane wave in a measurement point by sending waves from the source antenna to the reflector. However, the compact range method requires a hard-adherence and high-price reflector antenna. Also, in a low frequency band, i.e., hundreds MHz, size of the reflector is large so that installation and maintenance of the reflector are difficult, and in a high frequency band, i.e., over 100 GHz, the reflector is hard to be manufactured due to μm fabrication errors.
Meanwhile, there is a near field measurement method which measures a proximity field in a short-distance region and transforms the proximity field to a far-distance radiation pattern. The near field measurement method measures vertical polarization and horizontal polarization at a place apart from a probe by distance 3λ˜10λ, performs Fourier transforms of the vertical polarization and the horizontal polarization and acquires the far-field radiation pattern based on the Fourier transform values. However, mechanical instruments operating the probe should be precise, thus a lot of financial resources are needed.
Therefore, a method for measuring a radiation pattern of a large-size antenna is required without a high-price system or an additional anechoic chamber.
Accordingly, a method for measuring an antenna radiation pattern in a Fresnel region and transforms the measured data to a far-distance radiation pattern is suggested, where the Fresnel region having a small distance, which is a fraction of 2 L2/λ, is a intermediate region between a far-field region and near field region. During angles of an antenna to be measured are varied upward and downward, radiation patterns are measured and summed in a plurality of planes. In many published papers, it is verified that the Fresnel region measurement method has high accuracy.
However, the Fresnel region measurement method has several disadvantages in actual measurement environment. First, the Fresnel region measurement method can be applied to a far-field measurement system having a positioner that can vary angles of the antenna upward and downward. Unless, a high-price positioner should be brought.
In addition, when a large-size antenna is inclined downward in order to change angles of the large-size antenna having tens of kilograms (Kg), safety concerns such as a breakdown can occur due to weight of the antenna and the positioner. Moreover, when the angles of the antenna are varied upward and downward, undesirable reflection waves reflected from absorber adhered to a flat surface and a ceiling surface can be received, and thus measurement errors occur.
In order to solve the above problems, it is suggested that an antenna radiation pattern measuring system which can minimize the reflection waves from the flat surface and the ceiling surface by using a source antenna having high-directivity. However, in order to implement the above method, antenna to be measured should have high-directivity. Therefore, utilization of a standard antenna is low and it is not efficiency.